With the current trend of semiconductor devices scaling into the deep submicron region, design challenges that were previously minor issues have now become increasingly important. Where in the past, dynamic, switching power has been the predominant factor in CMOS digital circuit power dissipation, recently, with the dramatic decrease of supply and threshold voltages, a significant growth in leakage power demands new design methodologies for digital integrated circuits (ICs). The main component of leakage power is sub-threshold leakage, caused by current flowing through a transistor even if it is supposedly turned off. Sub-threshold leakage increases exponentially with decreasing transistor feature size.
Among the many techniques proposed to control or minimize leakage power in deep submicron technology, Multi-Threshold CMOS (MTCMOS), which reduces leakage power by disconnecting the power supply from the circuit during idle (or sleep) mode while maintaining high performance in active mode, is very promising. MTCMOS incorporates transistors with two or more different threshold voltages (Vt) in a circuit. Low-Vt transistors offer fast speed but have high leakage, whereas high-Vt transistors have reduced speed but far less leakage current. MTCMOS combines these two types of transistors by utilizing low-Vt transistors for circuit switching to preserve performance and high-Vt transistors to gate the circuit power supply to significantly decrease sub-threshold leakage.
There are multiple ways to implement MTCMOS in synchronous circuits. One method is to use low-Vt transistors for critical paths to maintain high performance, while using slower high-Vt transistors for the non-critical paths to reduce leakage. Besides this path replacement methodology, there are two other architectures for implementing MTCMOS. A coarse-grained technique uses low-Vt logic for all circuit functions and gates the power to entire logic blocks with high-Vt sleep transistors, as shown in FIG. 1. The sleep transistors are controlled by a Sleep signal. During active mode, the Sleep signal is deasserted, causing both high-Vt transistors to turn on and provide a virtual power and ground to the low-Vt logic. When the circuit is idle, the Sleep signal is asserted, forcing both high-Vt transistors to turn off and disconnect power from the low-Vt logic, resulting in a very low sub-threshold leakage current. One major drawback of this method is that partitioning the circuit into appropriate logic blocks and sleep transistor sizing is difficult for large circuits. An alternative fine-grained architecture, shown in FIG. 2, incorporates the MTCMOS technique within every gate, using low-Vt transistors for the Pull-Up Network (PUN) and Pull-Down Network (PDN) and a high-Vt transistor to gate the leakage current between the two networks. Two additional low-Vt transistors are included in parallel with the PUN and PDN to maintain nearly equivalent voltage potential across these networks during sleep mode. Implementing MTCMOS within each gate solves the problems of logic block partitioning and sleep transistor sizing; however, this results in a large area overhead.
In general, three serious drawbacks hinder the widespread usage of MTCMOS in synchronous circuits: 1) the generation of Sleep signals is timing critical, often requiring complex logic circuits; 2) synchronous storage elements lose data when the power transistors are turned off during sleep mode; and 3) logic block partitioning and transistor sizing is very difficult for the coarse-grained approach, which is critical for correct circuit operation, and the fine-grained approach requires a large area overhead.